Measurement of β+ emitters production cross sections of interest for range verification in proton therapy

Abstract

The uncertainty in the dose curve delivered during the treatment of tumors with protons, i.e. the position of the Bragg peak, makes it necessary to apply safety margins from 1% to 3% in the beam range. This means nearly 1 cm in a 30 cm deep tumor, which certainly limits the benefits of having a sharp Bragg peak. Reducing this uncertainty would allow a reduction of the treatment volume and thus result in a better utilization of the advantages of protons compared to conventional radiotherapy. Among the diferent techniques that are being proposed for range verification, some of which have already been clinically tested), the measurement of the distribution of β+ radioactive nuclei (known as PET isotopes) produced by the proton beam along its path allows detecting variations of only a few millimeters in the proton beam range. For this purpose, a better database of production cross sections of β+ emitters as function of the energy of the incident proton for a wide range of elements present in the body (mainly C, O, N, P and Ca) is needed, specially in the low energy range (up to 20 MeV). This applies, among others, to the commonly used long-lived PET isotopes 11C and 13N. The objective of this work is to measure the production cross sections of 11C and 13N in C, O and N at diferent energies from 7 to 18 MeV. For this purpose, stacks of targets containing these elements have been irradiated with the 18 MeV proton beam of the CNA cyclotron, and the 511 keV photons emitted in the annihilation of the positrons from the β+ decays of the isotopes of interest have been measured. This measurement was performed both with NaI(Tl) scintillator detectors and a PET/CT system. A total of six reaction cross sections have been sucessfully measured with uncertainties in the order of 15-20%. These have been compared to the data available in EXFOR showing an overall good agreement regarding the 16O(p,*)13N and 14N(p,*)11C reactions, and sizable differences in the case of the other reactions. Last, a series of improvements for future measurements have been identified and discussed.